This removes a lot of API from net/interfaces (including all the
filter types, EqualFiltered, active Tailscale interface func, etc) and
moves the "major" change detection to net/netmon which knows more
about the world and the previous/new states.
Updates #9040
Change-Id: I7fe66a23039c6347ae5458745b709e7ebdcce245
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>